In a press section of a paper machine, one of the most critical points with respect to runnability in the paper machine is the way the web behaves when it is separated from a centre roll. The separation of the web from the centre roll is especially important because the first free draw of the web is most often performed at this stage, that is to say the web is detached from the roll without support when it is especially susceptible to breaks if its properties and the factors affecting it have not been regulated correctly. The centre roll in the press has often constituted, as known per se, a critical point impairing runnability in the paper machine and it has limited the available maximum speed, even caused web breaks, and thereby reduced the efficiency of the paper machine.
Various factors affect the process of separation and the point of separation, i.e. the angle of separation, of the web. Among other things, properties of the web, linear loads of the press, draw differences in the press section, the wet end chemistry of the paper machine, the condition of the face of the centre roll, the operation of the doctor of the centre roll and the condition of jets used in connection with doctors, the selection of chemicals possibly used in connection therewith, all affect the separation point and, in addition, these factors are further partly dependent on one another. However, these factors influencing separation and indirectly also the operation of the doctor of the centre roll are not adequately known, because the effects of the factors influencing the process of separation may additionally appear with very different delays (from a second to weeks or months). Moreover, the effects may vary from one mill, process or moment to another. Since the process of separation has not been closely monitored, it has also not been possible to connect it with other process changes, and thus it has not been possible to establish right cause-to-effect ratios.
It is known from prior art to monitor the separation of the web by visually following an image of a video camera. However, this does not provide accurate information about the processes of separation but, instead, it gives random information because the monitoring of the separation process is based only on visual examination. Furthermore, the regulation measures possibly taken based on video camera monitoring have been mainly specific to a particular operator and based on the operator's personal analysis of the situation.
Thus, the monitoring of the separation of the web carried out by a video camera has not been accurate, and by means of it it has not been possible to monitor rapid changes in the separation process, nor has it been very simple to transfer the situations seen from it onto paper or into electric form to form proportional factors in a data collecting system of a paper machine, especially if normal sway of the separation process is large. Also, this arrangement provides the situation not farther away from the edge in the longitudinal direction of the roll but only at the edge of the web, which may cause miscalculations because the line of separation is not necessarily straight in the direction of the roll and, at the edge, there may occur disturbances that are not representative considering the whole web.